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SUMMARYA strain of Aspergillus amstelodami, which antagonized the growth of Mycobacterium tuberculosis, was shown to produce at least two antibiotic substances. A liquid medium was developed for their production, and cultural and assay procedures were defined. The antibiotics appear to be distinct from previously described derivatives of the true fungi and were named Amodin A and Amodin B. Both are active against certain Grampositive and acid-fast organisms, but Amodin B has the wider antibiotic spectrum and is active against some Gram-negative organisms and strains of Candida albicans. Amodin A was produced in surface and submerged cultures, but in better yield in the latter; it was extracted and prepared as a crude product. It appears to be a moderately heat-labile peptide; though non-toxic to mice it did not prolong their survival when infected with Staphylococcus aureus or M . tuberculosis a t the dosage of antibiotic used. Amodin B appeared only in surface cultures and was mainly present in the mycelium, from which it was liberated by dilute acid. It is relatively heatstable and not inactivated by proteolytic enzymes. Amodin A production appears to be linked with the conidial mode of sporulation and Amodin B with the perithecial mode.